1. Field of the Invention
The invention relates generally to electron guns and in particular to a method of fabricating a multiple gride electrode.
2. Description of the Prior Art
Electron guns are well known in the prior art, especially those electron guns utilized to generate an electron beam in traveling wave tubes (TWT). The present technology of traveling wave tubes may be generally divided into two types, either continuous wave (CW) mode or pulsed mode. There are some multi-mode TWTs which operate both as CW mode or pulsed mode and at various power levels. The pulsed and multi-mode TWT requires a more complex electron gun for its operation.
The electron gun used with a single mode CW TWT usually requires only a heater, a cathode and an anode within a vacuum envelope. Once the electron gun is turned on, the TWT operates at a single beam diameter and at a constant power level.
Continuous mode TWTs operate at a generally lower power level than the pulsed or multi-mode TWTs. For example, a pulsed TWT is capable of operating at power levels which are an order of magnitude greater than the same TWT operating in the CW mode.
A pulsed TWT requires additional structure, such as a control grid within the vacuum envelope of the electron gun. The control grid is a dish-shaped electrode having the same center of curvature as that of the cathode face. A small space, on the order of 0.003 inch, separates the control grid from the cathode face. The face of the control grid may have a symmetrical design composed of a series of thin vanes to allow the electron beam to pass through the grid as unobstructed as possible. The control grid is employed to interrupt the current flow, thereby turning the electron beam on and off, thus pulsing the TWT. To regulate the beam, a suitable positive potential, such as +400 volts above the cathode voltage, is applied to the control grid during the "on" period. To turn the electron beam "off", an appropriate negative potential, such as -400 volts below the cathode potential, is applied.
Sometimes another grid, called the shadow grid, is used in conjunction with the control grid. The shadow grid is interposed between the control grid and the cathode electrode and is maintained at the same potential as the cathode electrode. The shadow grid has the same configuration as the control grid. Its function is to form a shadow or shield for protecting the control grid, or any other associated grid, from receiving the "high interception current" from the electron beam. If the electron beam strikes the control grid or other subsequent grids, a high current will flow, thereby causing heating problems in that grid. If the control grid draws current from the electron beam, operating efficiency will be reduced. For the shadow grid to function properly, its structure must be the same as that of the control grid and both must be in perfect alignment.
In a dual mode TWT, i.e., operating at two power levels such as CW and pulsed mode, a third grid electrode, called a screen grid, is also used to control the electron beam. The screen grid is generally positioned between the shadow grid and the control grid and is separated from both of them by as small a distance as possible, depending on the operating voltages. Generally the separation distance is on the order of 0.003 inch. This grid electrode essentially regulates the diameter of the electron beam. By applying the proper negative voltage, relative to the cathode potential, such as -200 volts, a small electron beam is formed for CW operation. The electrical potential on the screen grid may be varied to control the beam current also. A positive voltage, relative to the cathode electrode, allows a much larger beam to be formed, i.e., more power, and the TWT can operate in the pulsed mode.
As is evident, the structure of the electron gun generally, and the grids in particular, determine the type of TWT (pulsed, CW, or multimode).
In TWTs requiring multiple grids, the alignment of those grids as well as the inter-grid separation determines to a great extent the operating characteristics of the TWT. Each of the grids has a vaned structure which is identical in shape to the other grids. The size and shape of grids are extremely important to the operating characteristics of the TWT. Also, the alignment of the individual vanes must be as precise as possible so that the second and third grids do not intercept the electron beam and draw energy away from it. The inter-grid spacing is also extremely important, since the greater the separation from the preceding grid the greater the voltage required on the succeeding grid to operate with consistent voltages gun to gun. Thus, the electrical characteristics would vary from one TWT to another if the spacing between grids were not maintained, making it impossible to predict electrical performance for a given TWT.
Current practice for fabricating grid electrodes for electron guns is to form each one individually. A single disc of molybdenum is placed on a press and a spherical dimple is formed. A vaned structure is then machined through the dimpled portion of the disc. Machining may be by the electrical discharge method or by photo-etching. Electrical discharge uses an electric arc to erode, or etch away, the unnecessary material from the disc, thereby leaving the desired vaned structure. After all the necessary grids have been fabricated, the grids are brazed onto their respective support members. The grid and support members are assembled, with ceramic and metallic spacers inserted between the grids. The entire assembly is aligned so that the grids are coaxial and so that the vanes of each of the grids overlap as nearly as possible. Generally, a ceramic pin is inserted through holes in the grids to keep them aligned.
The prior art process just described has many limitations. For example, the grids are generally not identical, since they are individually fabricated. If the dimples are not perfect then the requisite interdisc spacing cannot be maintained, thereby affecting the voltage requirements. Since the vanes are individually machined, they are normally not uniform and therefore are usually not in perfect alignment, thereby affecting the beam current. It is extremely difficult to etch identical vanes in different grids because the finished vanes are typically 0.001-0.002 inches wide. In addition, since each grid is individually formed, labor costs are high.